The invention relates to a circuit for the line synchronization in a picture display device, the circuit comprising a phase control loop including an oscillator for generating a local line-frequency signal, means for applying the local signal and an incoming signal comprising a line synchronizing signal to a phase comparison stage for comparing the phase between the signals applied and for applying a signal, which is dependent on the measured phase difference, to a loop filter for smoothing the signal and for applying the control signal obtained to the oscillator for controlling the frequency and/or the phase of the local signal.
Phase control loops of this type are generally used in picture display devices for continuously controlling the oscillator which generates the local line-frequency signal for the purpose of line deflection in a picture display tube or for the purpose of other circuits including a switch switching at the line frequency, for example the circuit for generating the extra high tension for the final anode of the tube or switched mode power supply circuits. The phase comparison stage is generally in the form of a multiplier stage for multiplying its input signals. For small phase differences such a stage is linear. However, a line phase control loop including a multiplier has the drawback that its lock range is wider than its capture range and that the control loop is unsuitable to capture over a wide frequency range only once. The control loop may also capture on a subharmonic of the line frequency. The line synchronizing circuit is therefore not readily suitable for picture display devices, for example monitors, with more than one repetition frequency over a wide range for the incoming line synchronizing pulses. The circuit described in European patent application No. 173,554 which is intended for more than one line frequency includes a facility for preventing capture errors.